The present invention relates to a process for preparation of 2,3,5-trimethylhydroquinone (hereinafter referred to as "TMHQ") and more particularly to a process for preparing TMHQ from 2,3,5-trimethylbenzoquinone (hereinafter referred to as "TMBQ") through its hydrogen reduction in the presence of platinum group metals deposited on an alkali metal-containing silica alumina as a support.
TMHQ is an important material for preparation of Vitamin E (.alpha.-tocopherol), and its demand has been rapidly increased in recent years. The quality of Vitamin E varies depending on that of TMHQ as the starting material. Thus, from a viewpoint of the quality of the product Vitamin E and also from an economic standpoint in the preparation thereof, it is required for TMHQ to have a high purity.
As is well known in the art, TMHQ can be prepared from various starting materials. Among these known methods is most predominant a method of preparing TMHQ through TMBQ.
One of the known methods to prepare TMHQ from TMBQ as the starting material is a method in which TMBQ is reduced with reducing agents such as sodium hydrogensulfite (Japanese patent application laid-open No. 108032/1974) and sulfur dioxide (Japanese patent publication No. 14455/1984). This method, however, has disadvantages in that the reducing agents are expensive, the yield is low, and impurities resulting from the reducing agent are readily incorporated in the product.
Another method is to reduce TMBQ with hydrogen in the presence of a catalyst (e.g., U.S. Pat. No. 2,229,573). This method is advantageous in that the reducing agent is inexpensive and is easy to separate because it is gas. Catalysts to be used in the method which have heretofore been known include catalysts comprising activated metals such as Raney nickel and Raney cobalt, renium, and platinum group metals such as palladium, ruthenium and rhodium as deposited on a support (Japanese Patent Publication No. 20285/1982). These catalysts can be easily separated from TMHQ and the solvent by filtration after completion of the reaction because they are solid. As described above, the hydrogenation method has advantages.
TMHQ to be used in preparation of Vitamin E which is to be utilized as a medicine or a food additive is required to have a purity as high as more than 99%. If the purity of TMHQ is lower than 99%, it is necessary to purify TMHQ. For this purification, for example, the following have been proposed: a method in which after completion of the reaction, a poor solvent is added to crystallize TMHQ from the reaction mixture (Japanese Patent Publication Nos. 26424/1976 and 20285/1982), and a method in which TMHQ once separated from the reaction mixture is recrystallized from a suitable combination of solvents (Japanese patent publication No. 20285/1982). These methods, however, have various disadvantages: for example, a large amount of solvents is used, the solvents used should be separated and recovered because they are mixed solvents, and if the purity of TMHQ is low, the amount of energy needed is increased. Moreover, as the purity of TMHQ is lower, the amount of TMHQ abondoned along with impurities is more increased, thereby leading to a reduction in the yield.
If the purity of TMHQ is sufficiently high, TMHQ can be used as the product without application of any purification operation, and even in a case where the purification is applied, the amount of energy is greatly decreased.
Thus the reaction selectivity in hydrogenation of TMBQ is a very important factor exerting great influences on the yield, purity and production cost of TMHQ, and it is of high industrial significance to develop a method whereby hydrogenation of TMBQ can be carried out with high selectivity.
In practical use of catalysts, the catalyst life is important from an economic standpoint and also from a viewpoint of operation; that is, it is essential for industrial catalysts that not only the activity and selectivity at an initial stage of use are high, but also the high activity and high selectivity are maintained over a long period of time. Also in hydrogenation catalysts for TMBQ, the catalyst life is important.
As supports for platinum metals which are to be used in hydrogenation of TMBQ, active carbon (U.S. Pat. No. 2,229,573), alumina (Japanese Patent Application Laid-Open No. 100430/1977) and silica, pumice and silicic acid (Japanese Patent Publication No. 23487/1976) are known. However, in hydrogenation of TMBQ in the presence of catalysts comprising platinum group metals deposited on the above supports, side reactions such as addition of hydrogen to carbon-carbon double bonds will occur, thereby leading to a reduction in the selectivity of the desired product TMHQ. High purification, therefore, is needed to obtain the product of high purity.
Moreover the service life of the above catalysts is not sufficiently long. Thus various attempts to lengthen the catalyst life have been made as in, for example, Japanese Patent Publication No. 42964/1979, but with no satisfactory results.